The increased demand for data communication and the remarkable growth of the Internet have resulted in increased demand for communication capability within metropolitan areas. There has also been an equally large increase in demand for communication capability between large metropolitan areas. Optical communication systems using a network of fiber optic cables are being developed and installed to meet the increased demand.
The data transmission capacity of fiber optic cables and fiber optic networks has been substantially increased as a result of wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM). Within WDM and DWDM systems, optical signals assigned to different wavelengths are combined (multiplexed) into a multiple wavelength signal for transmission over a single fiber optic cable or other suitable waveguide. A typical DWDM system modulates multiple data streams on to different portions of the light spectrum. For example, one data stream may have an assigned wavelength of 1534 nanometers (nm) and the next data stream may have an assigned wavelength of 1543.8 nm. The required spacing between assigned wavelengths is generally established by International Telecommunications Union (ITU) specifications. These spacings include 0.4 μm and 0.8 μm.
Demultiplexing, the reverse process of multiplexing, typically refers to separation of a multiple wavelength or multi-wavelength signal transmitted by a single fiber optic cable or other suitable waveguide into constituent optical signals for each wavelength. Each optical signal may be further processed to obtain the associated data stream or other information. Both multiplexing and demultiplexing are required for satisfactory operation of WDM and DWDM systems. Multiplexing and demultiplexing of optical signals in conventional DWDM systems are typically performed by two separate relatively expensive and often difficult to manufacture optical devices.
Various types of optical switches and techniques are currently used in optical communication systems. Many currently available optical switches are based upon optoelectric and electrooptic conversion of light signals and electrical signals within the associated optical switch. One type of presently available optical switch includes a matrix of thermooptic switching elements interconnected by waveguides formed on a silica substrate. Switching of light signals is accomplished by the use of thin film heaters to vary the temperature of the switching elements. Electrical circuits are also provided to supply switching current to the heaters. A heat sink may be provided to dissipate heat caused by the switching operations. One example of such switches is shown in U.S. Pat. No. 5,653,008.
Various types of optical signal amplifiers, wavelength division demultiplexers, optical switches, wavelength division multiplexers and techniques are currently used in optical communication systems. Optical signal amplifiers, wavelength division multiplexers and demultiplexers and other components associated with optical communication systems that transmit multiple wavelength light signals typically function best when respective signal levels for the multiple wavelength optical signals are substantially equal with each other. A substantial variation in signal level of multiple wavelength optical signals can result in an undesirable signal to noise ratio and resulting poor performance.
Multiple wavelength optical signals are normally collectively amplified by a light amplifier. The amplification factor of many light amplifiers is dependent upon the wavelength of each optical signal. Therefore, the amplification factors for multiple wavelength optical signals varies depending on the specific wavelength of each signal. The resulting difference between signal levels for multiple wavelength optical signals amplified by a single amplifier is often relatively small. However, when a large number of light amplifiers (ten or more) are used in a fiber optic communication system, the variation in signal levels becomes cumulative and may result in unsatisfactory lowering of associated signal to noise ratios. Therefore, variable optical attenuators are often provided at the input stage and/or output stage of light amplifiers in both large metropolitan communication systems and long distance fiber optic communication systems to adjust signal levels or intensity of multiple wavelength light signals to maintain a desired signal to noise ratio.
Variable optical attenuators are often included in optical communication systems to maintain a desired signal level for each optical signal or wavelength. Examples of variable optical attenuators (VOA) include natural density filters that are often used to suppress the amount of light depending on wavelength characteristics. Other variable optical attenuators include mechanical devices that position a glass substrate so that light signals may be attenuated by varying the position of the glass substrate. Still other variable optical attenuators attenuate light signals by rotating the polarization of each light signal as it passes through a Faraday element.